Solder is used as an adhesive to attach optical fibers to a circuit board. Efforts to model the motion of an optical fiber during the wetting and solidification of the adhesive solder droplet are discussed in a reference by Adam Powell et al. entitled “Mechanism Of Motion Of An Optical Fiber Aligned By A Solder Droplet”, Mat. Res. Soc. Symp. Proc., Vol. 531, 1998. The extent of motion is determined by several competing forces, during three stages of solder joint formation. First, capillary forces of the liquid phase control the fiber position. Second, during solidification, the presence of the liquid-solid-vapor triple line as well as a reduced liquid solder volume leads to a change in the net capillary force on the optical fiber. Finally, the solidification front itself impinges on the fiber. Publicly-available finite element models are used to calculate the time-dependent position of the solidification front and shape of the free surface.
Unfortunately, during the melting and solidification of the solder, significant motion of the fiber is observed to occur, decreasing the transmission efficiency. Because the capillary force on the fiber scales as its diameter and its stiffness as its diameter cubed, a very thin fiber is likely to be significantly deflected by the capillary force. Furthermore, as the solid joint solidifies, its volume changes, and with it the shape of the liquid surface and the magnitude of the capillary force on the fiber. Finally, as the solidified solder makes contact with the fiber, the shrinkage during cooling pulls the fiber further out of alignment. The disclosure of the Adam Powell reference is hereby incorporated herein in its entirety by reference.